Page 262 - Introduction to Paleobiology and The Fossil Record

ORIGIN OF THE METAZOANS 249
walled tube possibly of an unsegmented
worm.
Many of the Tommotian animals are form
taxa (that is, named simply by their shapes)
because the biological relationships of most
cannot be established and often there are few
clues regarding the function and signifi cance
of each skeletal part. Most are short-lived and
have no obvious modern analogs. Two groups
are common – the hyolithelminthids have
phosphatic tubes, open at both ends, whereas
the tommotiids are usually phosphatic, cone-
shaped shells that seem to belong in bilater-
ally symmetric sets.
Discoveries of near-complete examples of
Microdictyon-like animals from the Lower
Cambrian of China have helped clarify the
status and function of some elements of the
Tommotian fauna. These worms have round
to oval plates arranged in pairs along the
length of the body, which may have provided
a base for muscle attachment associated with
locomotion. As noted previously, many of the
small shelly fossils are probably the sclerites
of larger multiplated worm and worm-like
animals (Box 10.5).
The Meishucunian biota
The Meishucunian Stage of South China has
yielded some of the most diverse Tommotian-
type assemblages in strata of Atdabanian age
(see Appendix 1). Qian Yi and Stefan Bengt-
son (1989) have described nearly 40 genera
that belong to three largely discrete, succes- happened just as fast as the fossils suggest,
sive assemblages through the stage. First,
the Anabarites–Protohertzina–Arthrochites
assemblage is dominated by tube-dwelling
organisms such as Anabarites; the Siphongu-
chites–Paragloborilus assemblage contains
mobile mollusk-like and multiplated organ-
isms together with some tube-dwellers and
possible predators; whereas the Lapworth-
ella–Tannuolina–Sinosachites association has
mainly widespread multiplated animals.
Many of these fossils are known from
Lower Cambrian horizons elsewhere in the
world, highlighting the global distribution
of many elements of the fauna. However,
the three “community” types are rather
mysterious, and probably represent different
ecosystems, but it is hard to speculate
further.
Distribution and ecology
Although it is still unclear whether many of
the Tommotian skeletons are single shells or
single sclerites and the autecology of most
groups is unknown, the assemblage was cer-
tainly the first example in evolution of a skel-
etalized benthos. Very few of the Tommotian
skeletal parts exceed 1 cm; nevertheless many
shells were the armored parts of larger worm-
like animals. And both mobile and fi xed forms
occurred together with archaeocyathans and
non-articulate brachiopods. The microben-
thos of the Tommotian was succeeded by a
more typical Cambrian fauna, dominated by
trilobites, non-articulate brachiopods, mono-
placophoran mollusks and primitive echino-
derms together with the archaeocyathans
during the Atdabanian Stage (Fig. 10.14).
Cambrian explosion
The Cambrian explosion suddenly generated
many entirely new and spectacular body plans
(Box 10.6) and coincides with the appearance
of the Bilateria over a relatively short period
of time (Conway Morris 1998, 2006). This
rapid diversification of life formed the basis
for Stephen Jay Gould’s bestseller, Wonderful
Life (1989), which took its title from the
Frank Capra 1946 fi lm It’s a Wonderful Life.
The rapid appearance of such a wide range of
apparently different animals has suggested
two possible explanations. The “standard”
view is that the diversifi cation of bilaterians
and that some reasons must be sought to
explain why many different animal groups
apparently acquired mineralized skeletons at
the same time. An alternative view arose after
initial molecular studies had suggested that
animals diverged some 800 myr before the
beginning of the Cambrian (e.g. Wray et al.
1996). If these molecular views were correct,
then the absence of fossils of modern animal
phyla through the Proterozoic would have to
be explained by an interval of cryptic evolu-
tion of probable micro- and meioscopic organ-
isms, living between grains of sand, operating
beneath the limits of detection prior to the
explosion (Cooper & Fortey 1998). Greater
refi nement of Cambrian stratigraphy, the tax-
onomy and phylogeny of key Cambrian taxa
and their relative appearance in the fossil